Cellulose ethers are conventionally prepared in two stages, viz (1) an alkalization stage wherein an alkali is reacted with cellulose to prepare an alkali cellulose, and (2) an etherification stage wherein an etherifying agent is reacted with the alkali cellulose to form the cellulose ether product. Dispersing agents or solvents are sometimes added to one or both stages to obtain better mixing. Typically cellulose in finely divided state is reacted in a first stage with an alkaline solution, e.g. an alkali metal hydroxide, the alkaline solution generally being sprayed upon the cellulose fiber and reacted therewith in an alkalization reaction to form the alkali cellulose. The alkali cellulose is reacted in a second stage with an etherifying agent in an etherification reaction to form the cellulose ether.
In the alkalization reaction the reactor is vigorously stirred, e.g. in a heterogenous reaction medium, to mix the alkali and cellulose as uniformly as possible, and generally the reaction is carried out at low, often ambient, temperature. The oxygen partial pressure is controlled to suppress or promote polymer degradation. In general, the alkalization reaction is conducted at temperatures up to about 30.degree. C.
The etherification reaction is conducted, e.g. in a heterogenous reaction medium, by heating the alkali cellulose together with the etherifying agent to produce the cellulose ether. The etherification reaction is generally conducted at temperatures ranging from about 30.degree. C. to 100.degree. C. with simultaneous vigorous stirring of the reactants to obtain as uniform substitution and good yield of the cellulose ether as possible with as little consumption as possible of the etherifying agent.
In conducting the alkalization and etherification reactions, the two stages constituting the process have conventionally been conducted in the same reaction vessel, or in separate reaction vessels. In either, the reactants are sequentially added. In the former, after formation of the alkali cellulose by reaction between the alkali and cellulose, the etherifying agent is then added to the same vessel to convert the alkali cellulose intermediate to cellulose ether. In the latter, the alkali cellulose is passed from the first vessel to the second vessel, or zone of the series to which the etherifying agent is added to form the cellulose ether. The two stage operation better lends itself to a continuous operation. All of the reactants might also be added with the cellulose to a single reaction vessel and the reactants treated, with stirring, to carry out the process in a batch operation. The batch operation also requires two steps; a first step at low temperature to produce the alkali cellulose and then, in a second step, the operation is conducted at higher temperature to produce the cellulose ether.
In the production of cellulose ethers from these heterogenous reaction mixtures, beginning with the initial step of preparing the alkali cellulose, an inherent difficulty is presented in that the volume of the cellulose fiber relative to that of the alkali metal hydroxide is massive for which reason it is extremely difficult if indeed possible to properly, much less completely disperse the alkali metal hydroxide upon the cellulose. A tremendous surface area is presented by the cellulose, and the hydroxide reaction sites on the anhydro-D-glucose units of the cellulose with which the alkali metal hydroxide must react to form the alkali cellulose are widespread. It is nonetheless essential to achieve a high degree of uniformity of substitution of the hydrogen of the hydroxyl groups of the anhydro-D-glucose units of the cellulose by the alkali metal. The substitution must be as complete and uniform as possible to obtain a cellulose ether with a minimum residue of insoluble components. In order to obtain better dispersion of the alkali metal hydroxide within the alkaline solution, increasing the amount of the alkali metal hydroxide proves ineffective because this leads to increased side reaction between the alkali metal hydroxide and the etherifying agent requiring, inter alia, excessive amounts of the etherifying agents with consequent debits to the process. Dilution of the alkaline solution with water to obtain better alkali metal dispersion, on the other hand, is likewise ineffective because yields are also adversely affected. Moreover, the use of organic solvents to obtain better dispersion of the alkali presents limitations in that the solvents can be absorbed into the cellulose and cannot be displaced by the alkali, this effectively reducing the amount of cellulose available for reaction. Further, the solvent can react with the etherifying agents, thus competing with reaction between the etherifying agent and the cellulose so that the yield of cellulose ether is reduced. Furthermore, the addition and use of any diluent or solvent in the process must be recovered, and hence can constitute a burden unless the beneficial effect can offset this burdensome consequence.